1. Field of the Invention
The invention relates to a radiogenic unit and to its cooling system, enabling high power to be obtained while at the same time keeping the external casing of the apparatus cool.
Such a unit is used especially in radiology instruments.
2. Description of the Prior Art
At present, a radiogenic unit comprises an X-ray tube that is placed in a protective casing and is supplied with high voltage. The X-ray tube essentially comprises a cathode and an anode enclosed in a glass chamber under vacuum. The cathode is formed by a thermoelectronic emitter, such as a tungsten filament, that is housed in a metallic focussing element and that, when heated, emits an electron beam focussed on the anode. This anode is generally constituted by a massive disk of graphite covered with a layer of a material with a high atomic number emitting X-rays when it is bombarded by an electron beam. The anode is taken to a high positive potential (of several tens of kilovolts) with respect to the cathode, and the electrical field thus created between the two electrodes accelerates the electrons that are emitted by the cathode and that strike the anode on a small surface or zone of impact of said electron beam on the anode, which constitutes the focal spot of emission of the X-radiation. The high voltages that have to be applied to the electrodes are given by supply devices called high-voltage supply devices, such as the one described in U.S. Pat. No. 5,003,452 filed on behalf of the Applicant. A supply device such as this comprises inter alia a high voltage transformer connected to a voltage-doubling rectifier circuit and a heating transformer connected to the cathode.
Furthermore, the energy dissipated to produce the electron beam is converted partly into X-rays but above all into heat. This is why the X-ray tubes are positioned inside an insulating casing in which there flows a coolant fluid, generally electrically insulating oil. For a fixed anode tube, the heating of the anode is proportional to the mean power. Now, the anode on the one hand as well as the heating transformer on the other hand are electrically insulated, in bipolar mode, for a voltage equal to half of the maximum voltage. However, the cooling of the tube is limited by the fact that the two electrodes are insulated by oil whose thermal conductivity is relatively low, about three thousand times lower than that of copper. Owing the heating of the casing and the insulation oil inside, presently used radiogenic units work with limited mean power values.
The object of the present invention is to solve these problems by proposing a cooling of the anode that is speedier and more efficient, and an insulation of the voltage of the cathode with respect to the electrical ground or frame in a volume that is reduced as compared with prior art radiogenic instruments.